Innate immunity is an ancient and conserved mechanism that relies on preformed receptors to detect conserved molecular signals triggering downstream broad-spectrum defense responses. It forms the first line of defense against microbial pathogens and has been extensively described in animal systems. Recent work has shown that there are striking similarities between animal, insect and plant recognition systems operating in innate immunity. Innate immunity in plants can be triggered by a wide variety of molecules including (glyco)proteins, peptides, carbohydrates and lipids. Recently, oligosaccharide elicitors such as chitin and oligogalacturonide have received attention as important signals in plant defense. There is evidence that chitin oligomers are generated from fungal cell walls and insect exoskeletons during infection and that these molecules elicit a variety of defense responses. Our long-term goal is to elucidate the chitin perception and signaling pathway leading to plant defense and characterize its role in innate immunity. We have identified two chitin responsive genes, atl7and crpl, which directly impact plant defense responses. Using these genes as a starting point, we propose to begin characterizing the chitin elicitor response pathway. The specific aims of the project include: 1) to determine the functions of atl7 and crpl; 2) to determine where atl7 and crpl are expressed and localized in the plant cell; and 3) to determine the effectors of atl7 and crpl in Arabidopsis thaliana. The characterization of these genes will provide crucial initial information in order to start filling in the molecular components of the chitin signaling pathway. In the longer term, analysis of chitin signaling in Arabidopsis may provide some insight into the evolution of elicitor-mediated responses and their roles in both mammalian and plant-induced defense. Because of the sequence similarity among components of innate immunity in organisms as diverse as plants, Drosophila and mammals (e.g., Toll-like receptors, pelle-like kinases, ankyrin-repeat containing proteins), there is precedence to suggest that some signaling pathways in defense have ancient origins. This data will ultimately shed more light on the mechanisms behind innate immunity in both animal and plant systems.